The United States has belatedly reached the same conclusion as Britain and
France that Syrian regime used chemical weapons. Alex Spillius
examines what changed its mind.

But it has so far shown the same caution as its allies when it comes to offering public justification for its decision to arm the rebels and potentially embroil the country in yet another Middle Eastern conflict.

The world will have to wait for a similar moment to Colin Powell's exhibition of evidence of Iraq's weapons of mass destruction programme at the United Nations in 2003. As yet, there is no dossier to back up Washington's claims.

The White House has said that "we have a high-confidence assessment that chemical weapons have been used on a small scale by the Assad regime". It has passed its findings on to UN inspectors, Russia –Syria'skey ally at the UN – and other leading powers.

US officials said its case had been built from intelligence reports that Syrian officials had planned and executed chemical weapons attacks. It had received numerous eyewitness descriptions of symptoms of victims and, most importantly, laboratory analysis of "physiological samples" obtained from several individuals who revealed exposure to the nerve gas sarin. The New York Times reported that CIA officers had gathered blood, urine and hair samples from two Syrian rebels – one dead and one wounded – fighting government troops in March that tested positive for sarin.

Ben Rhodes, deputy national security adviser to the president, said that since the administration first went public with concerns about chemical weapons in April, the US intelligence community had been "exhaustively reviewing information".

It had focused on four incidents, he said: two in Aleppo, one in Homs and one in Adra near Damascus on May 23.

Evidence about the latter incident was it appears gathered by the French.

The Adra attack was reported on exclusively by Le Monde, and a week later Laurent Fabius, France's foreign minister, said tests on chemical samples taken from Syria proved beyond doubt that sarin had been used by the regime. French foreign ministry sources yesterday confirmed that the intelligence had been passed on to the Americans through the UN.

The French intelligence may have tipped the balance of opinion within the administration towards arming the rebels. But if the intervention goes wrong, the American public may want to see the verification for themselves.